1. Field of the Invention
The present invention relates to delay blasting caps, and more particularly to electric and non-electric blasting caps having a carrier-supported delay charge of an exothermic-burning composition adjacent to a priming charge of a heat-sensitive detonating explosive.
2. Description of the Prior Art
The art of delay blasting is practiced widely in underground and open-work blasting operations as a means of improving rock fragmentation and displacement; providing greater control of vibration, noise, and fly rock; reduce the powder factor; and reduce blasting costs. Short-interval or millisecond-delay caps (e.g., caps having nominal delay times of no greater than about 1000 milliseconds) and long-interval delay caps (e.g., those having nominal delay times of greater than about 1000 milliseconds) have been designed around the needs of different blasting requirements. At the present time, millisecond (MS) delays are the most widely used delay caps for quarry, open-pit, and construction projects, and they are also used in underground mines for multiple-row slabbing blasts, stope blasts, and other production blasts where rows of holes are breaking to a free face. Typically, MS delay blasts will move rock farther away from the face than long-interval delay blasts because of the interaction between successive boreholes fired at the shorter delay intervals. The nominal time interval between periods of successive caps in an available series often is as low as 25 milliseconds for lower-delay-period MS caps, although it can be up to 100 milliseconds for higher-delay-period MS caps, and up to about 500-600 milliseconds for long-interval delay caps.
An important prerequisite to successful delay, especially MS delay, blasting is that the delay times of a number of caps of stated delay rating be as uniform as possible from cap to cap. Desirably, the variation from the nominal value of the delay times of a given group of caps of assigned nominal delay time should be small enough that no less than 8 ms elapse between the firing of caps of any two consecutive periods. This would mean a maximum variation of .+-.8 ms for caps in the 25-ms; .+-.21 ms for those in the 50-ms; and .+-.46 ms for those in the 100-ms interval series. Without good uniformity, it is difficult to achieve a desired fragmentation, vibration reduction, etc. as expected from a given delay pattern.
In delay blasting caps, the delay interval, i.e., the time between the application of electrical or percussive energy and the detonation of the cap, is provided by the interposition of a delay charge of an exothermic-burning composition between the ignition system and the priming charge of heat-sensitive detonating explosive. The burning rate of the delay composition and the length of its column determine the delay interval. While in some caps the delay charge is pressed, without any surrounding element, directly into the cap shell over the priming charge, usually the delay charge is housed within a heavy-walled rigid carrier tube, e.g., as shown in U.S. Pat. Nos. 2,999,460 (FIG. 1) and 3,021,786 (FIG. 2). Use of a carrier tube is desirable in that the smaller loading (i.e., weight of charge per unit length) associated therewith allows the charge to be lengthened (to provide longer delays) without concomitantly increasing the total weight of the charge to a level which may be sufficient to burst the cap shell and deleteriously affect the delay timing.
The delay carriers heretofore known in the art have been mostly heavy-walled metal, usually lead tubes, although the aforementioned U.S. Pat. No. 2,999,460 states that the heavy-walled carrier shown in FIG. 1 therein is, for example, lead or plastic tubing. Also, U.S. Pat. No. 2,771,033 describes a core of a delay composition surrounded by a flexible textile envelope; and U.S. Pat. No. 2,773,447 describes the delay core surrounded by a thin paper- or textile-covered sheath that melts as the delay composition burns.